Conventionally, the embedding technique of the silicon carbide semiconductors is disclosed in, for example, Materials Science Forum P131–134, Vols. 264–268, 1988.
In this document, while various samples were employed, the trench inclination angles of which are approximately 50 degrees, and these samples own various sorts of trench widths, the aspect ratios of which are equal to or smaller than 1, such experiments have been carried out. That is, the epitaxial growing processes have been performed under such a condition that growth temperatures are 1480° C. and 1620° C., and C/Si ratios are 1.2 and 4.0.
In the above document, while the growth shapes of the epitaxial layers may strongly depend upon the C/Si ratios rather than the growth temperatures, in such a case that the C/Si ratio becomes low, the growth mode of the epitaxial layer becomes the surface reaction rate controlling, and thus, becomes the facet plane growth in which a plurality of facet faces appear, whereas in the case that the C/Si ratio is high, the growth mode of the epitaxial layer becomes the vapor phase diffusion rate controlling.
Also, as conventional techniques for embedding inner portions of trenches by epitaxial films, a large number of conventional techniques related to silicon semiconductors have been disclosed. For instance, Japanese Patent Publication No. 3424667 discloses the following technical idea. That is, rough faces and crystal defects as to inner planes (namely, side planes and bottom planes) of a trench, which occur in a trench etching step, are thermally processed in a non-oxidation atmosphere so as to smooth the inner planes of the trench, and thus, the crystalline of the embedded layer may be improved.
Also, Japanese Patent Application Publication No. 2003–218038 discloses the effect capable of round-shaping a corner portion of a trench bottom portion, while a stress of the trench bottom portion is relaxed and lowering of a growth rate in the trench bottom portion can be prevented. This conventional technique may be understood in the following different way when the view point of the described fact is changed. That is, when such a region is small where the growth of specific planes (for example, both bottom plane and side plane) joins with each other, a stress is increased. However, since a corner is rounded (namely, specific planes do not appear), a region where the growth of the specific planes joins with each other is made large so as to relax a stress. In other words, such a fact that the corner portion of the trench bottom plane is shaped as a round corner may constitute a necessary condition required for relaxing a crystal stress during growth, and for forming an embedding layer having a better crystalline characteristic.
However, in the case that the N type channel layer and the P+ type gate region, which will be formed inside the trench in the silicon carbide trench J-FET disclosed in JP-A-2003-69041, such a problem occurs which cannot be solved by the above-explained conventional techniques. More specifically, in such a case that the aspect ratio of the trench is equal to or larger than 2, and the inclination angle is substantially vertical, this problem may occur which cannot be solved by the conventional techniques.
That is to say, the below-mentioned problem is provided as to the embedding process of the trench in which the subject inclination angle is vertical and the aspect ratio is high. Namely, under the condition of the high C/Si ratio, since the supply of the material gas into the trench is decreased, as compared with that of the non-trench portion, when the N type channel layer is formed, the shape thereof is brought into an overhang state, whereas when the P+ type gate region is formed, the cavity is produced in the trench. In addition, since the growth rate of the trench inner portion is necessarily lower than that of the non-trench portion, even when the P+ type gate region could be embedded without an occurrence of such a cavity, there is another problem that the removing film thickness of the unnecessary epitaxial film in the etch-back step after the embedding step may surely become larger than the depth of the trench.
Also, the facet plane growth may occupy the dominant position under the low C/Si ratio condition. However, the above documents do not clarify a difference between the growth rates depending upon the face orientation. Furthermore, the above documents never disclose any clear indications how to increase the growth rate of the trench inner portion, as compared with that of the non-trench portion, and also how to decrease the removing film thickness of the unnecessary epitaxial film in the etch-back step after the embedding process.
Also, smoothening of an inner face of a trench in a silicon technique is realized by utilizing a feature of an Si crystal fluidity (refer to, for example, JP-A-11-74483) during a thermal processing operation, which can effectively reduce surface concaves/convexes and crystal defects. Moreover, at the same time, a corner portion of a trench may be rounded, and while a trench width is not substantially changed, a trench opening portion may be enlarged, so that a supply of material gas into the trench can be effectively increased. Since a trench bottom portion has no corner portion, the crystalline of the embedding layer may become superior, and furthermore, electric field concentration occurred when a semiconductor element is turned off may be suppressed due to the shape thereof.
On the other hand, since there is no liquid phase state in silicon carbide, there is no fluidity. As a result, in silicon carbide, in order to remove the rough faces of the trench inner planes (side plane and bottom plane) and the crystal defect, which occur in the trench etching step, the silicon carbide crystal region containing the rough faces and the crystal defect must be removed by the etching process. However, there are other problems. As to silicon carbide, there are no clear indication as to a wet etching fluid and a dry etching condition, which are capable of effectively removing a trench etching damage. Also, a sacrifice oxidation owns such a problem that an oxidation rate is low and a long oxidation time is required.
As other effective etching techniques, there are a hydrogen etching process and an HCL etching process, which are generally utilized as a growth pre-process for plane epitaxial growth. Normally, in these techniques, even in Si, the etching processes are employed at a temperature of approximately 1000° C., and it is easily conceivable that even in silicon carbide, a similar mechanism is operated. However, the mechanism as to Si is completely different from the mechanism as to silicon carbide.
In other words, in Si, the fluidity of Si is quickened in a thermal process operation at a temperature of approximately 1000° C., so as to recrystalline Si, so that the rough faces and the crystal defects are removed. As a result, when a non-oxidation atmosphere is employed as the atmosphere, then a sufficiently large effect may be obtained. On the other hand, since silicon carbide is constructed of two chemical elements namely, C and Si, and owns no fluidity, the respective elements must be removed from a substrate surface. Normally, the C element as a carbon is reacted with high temperature hydrogen so as to be removed as hydrocarbon (CxHy), whereas the Si element as a silicon is removed by vaporization operation under reduced pressure. As a consequence, in the etching process of silicon carbide, either a hydrogen atmosphere at a temperature equal to or higher than 1300° C. under reduced pressure or such a hydrogen atmosphere to which HCl equal to or higher than 1300° C. has been added under the normal pressure is necessarily required. As a result, when the thermal process is carried out in the hydrogen atmosphere to which HCl is not added under the normal pressure, only C elements are removed, and removing of Si elements is disturbed. As a consequence, a so-called “Si droplet” phenomenon may occur where only Si elements are left in the substrate surface and are condensed. This “Si droplet” phenomenon never occurs in the Si technique.
While the silicon carbide etching process owns such a feature, in the case that this silicon carbide etching process is applied to the trench shape, no detailed discussion example has been so far made. Therefore, there are various problems: a difference in etching rates depending upon face orientation of etching planes is not clarified; how to round a trench corner portion while preventing the surface reaction rate controlling by which a plurality of facet planes are produced; and also, the condition thereof is not clarified. Furthermore, in the case that the trench etching damage region of the trench inner planes (side plane and bottom plane) has been removed, there is no clear indication as to the reducing effects for reducing the surface convexs/concaves, and the crystal defects.